1. Field of Invention
The present invention generally relates to optical filters and more particularly to optical filters for optical fiber communication networks.
2. Description of the Related Art
The Synchronous Optical Network (SONET) standard defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium Currently optical communication is accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber data streams on different portions, a.k.a. channels, of the light spectrum.
The spacing between channels is constantly being reduced as the resolution and signal separation capabilities of multiplexers and de-multiplexers are improved. Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 0.4 nm, i.e., 50 GigaHertz (GHz). At this channel separation as many as 128 channels may be supported in C-band alone. Each channel is modulated on a specific center frequency, within the range of 1525-1575 nm, with the center frequency of each channel provided by a corresponding one of 128 semiconductor lasers. The modulated information from each of the semiconductor lasers is combined (multiplexed) onto a single optic fiber for transmission. As the length of a fiber increases the signal strength decreases. To offset signal attenuation erbium doped fiber amplifiers (EDFAs) are used at selected locations along the communication path to boost signal strength for all the channels. At the receiving end the processes is reversed, with all the channels on a single fiber separated (demultiplexed), and demodulated optically and/or electrically.
Optical filters play important roles in handling these optical communications for the telecommunications industry. They perform wavelength multiplexing and demultiplexing of the 128 or more optical channels. They may also be used to gain scale EDFAs by flattening their gain profile.
The requirements for optical filters used for any of these applications are very demanding. The close spacing between the channels in a WDM, makes it desirable to design a WDM with flat pass bands in order to increase the error tolerance. This is primarily because the center wavelength of a transmitter slips with temperature. Further, the cascading of the WDM stages causes the pass bands to become narrower at each WDM down the chain. Therefore, the larger the pass bands the greater the shift tolerance of the channel.
Various devices, such as multi-stage band and comb splitters, have been proposed to fill these new demanding requirements and none are fully satisfactory. In a multi-stage band splatter, the first stage makes a coarse split of two wavelength ranges, and subsequent stages make finer and finer splits of sub-bands within each of the wavelength ranges. In a multi-stage comb splitter, the first de-multiplexing stage filters out two interlaced periodic sets of relatively narrow band passes and the subsequent stages employ wider band pass periodic filters until the individual channels are de-multiplexed. In either case, noise and inter-channel interference are limiting factors in the handling of increasingly narrow band pass requirements. Multi-layer thin-film filters can be used to construct optical filters in bulk optics, but they are undesirable because of an increase in the number of layers for narrow channel spacing, precision of manufacture and expense associated with increasingly narrow band pass requirements. Further, dispersion will become a major issue as channel spacing decreases. Especially at 50 GHz channel spacing, dispersion of thin film filter is so big that it can not be used for OC-192 signal (10 Gbit/sec). Mach-Zehnder interferometers have been widely employed, but they have a sinusoidal response, giving rise to strongly wavelength dependent transmission and a narrow rejection band. Other designs have encountered a variety of practical problems.
Accordingly, there is a need for new optical filters for optical multiplexing/demultiplexing and other optical applications.
The present invention provides an optical device that can be used in a range of telecommunications applications including optical multiplexers/demultiplexers and optical routers. The optical device splits and combines optical signals of frequency division multiplexed optical communication channels which are evenly spaced apart in frequency from one another. The optical device includes a first filter and a second filter. The first filter splits and combines odd and even channels depending on the propagation direction of the optical signal. The first filter exhibits complementary phase retardations corresponding with odd integer multiples of half a wavelength for each center wavelength associated with a selected one of the odd and even set of channels and with integer multiples of a full wavelength for each center wavelength associated with a remaining one of the odd set and the even set. The second filter couples with the first filter to filter the odd and even sets of channels with phase retardations complementary to those experienced by the odd and even set of channels in the first filter. This complementary filtration has the effect of reducing dispersion in the device.
In an alternate embodiment of the invention the optical device interfaces with a first port communicating odd and even channels and with second and third ports communicating odd and even channels respectively. The optical device includes a linear polarizer, a first filter and a second filter. The linear polarizer couples to the first port for linearly polarizing optical signals. The first filter has a first free spectral range substantially corresponding with the channel spacing between adjacent odd or even channels. The first filter couples with the linear polarizer for splitting and combining odd and even channel sets depending on a propagation direction. The first filter operates as a full waveplate to a selected one of an odd channel set and an even channel set and as a half-waveplate to a remaining one of the odd set and the even set. The second filter optically couples with the first filter and the second and third ports. The second filter has a second free spectral range substantially corresponding with the channel spacing between adjacent odd or even channels. The second filter couples with the first filter for optical processing of odd and even channels therewith. The second filter operates as a half-waveplate to the selected one of the odd set and the even set and as a full waveplate to the remaining one of the odd set and the even set. In an alternate embodiment of the invention a method for splitting and combining optical signals is disclosed. The method includes subjecting odd and even channel sets to a first set of phase retardations corresponding with odd integer multiples of half a wavelength for each center wavelength associated with a selected one of the odd set of channels and the even set of channels and corresponding with integer multiples of a full wavelength for each center wavelength associated with a remaining one of the odd set and the even set. The method also includes subjecting odd and even channel sets to a second set of phase retardations corresponding with integer multiples of a full wavelength retardation for each center wavelength associated with the selected one of the odd set and the even set and corresponding with odd integer multiples of a half wavelength retardation for each center wavelength associated with the remaining one of the odd set and the even set.
Associated means are also disclosed.